1. Field of the Invention
The present invention relates to a vibration isolator that can isolate a load such as a table platform from a surface such as a floor of a building.
2. Background of the Invention
It is sometimes desirable to prevent relative movement between two surfaces. For example, integrated circuit are typically fabricated on a platform with photolithographic equipment. The location of directed light used to align and fabricate the integrated circuit must be very accurate.
The table is typically placed on the floor of a clean room. The floor may undergo vibrational movement that can be transferred to the table. The vibration may cause a displacement of the table which reduces the accuracy of the fabrication process.
Some tables incorporate vibration isolators to reduce or prevent the floor vibration from being transferred to the table. U.S. Pat. No. 5,000,415 issued to Sandercock and assigned to the assignee of the present invention, Newport Corp., discloses a vibration isolator that has an active isolator assembly which actively isolates a load from a floor. The active isolator assembly includes a plurality of piezoelectric actuators which can vary the distance between the load and the floor surface to compensate for movement in the floor. For example, the floor may oscillate so that the floor surface moves toward the load and away from the load. When the floor moves toward the load the piezoelectric actuators contract so that the motion of the load relative to inertial space is reduced compared to that of the floor. Likewise, when the floor moves away from the load the actuators expand.
The active vibration isolator disclosed in the Sandercock patent includes a sensor that senses the movement of the floor and circuitry to provide a control loop to synchronize the contraction/expansion of the actuators with the movement in the floor. Sandercock also discloses the use of sensors which sense the velocity of the load to provide a feedback loop that is coupled to the feedforward loop.
The piezoelectric actuators and control loops are capable of isolating the load for relatively low frequencies. To roll off high frequencies, Sandercock employs an elastomeric mount that is interposed between the load and the actuators. The elastomeric mount has a resonant frequency that varies with the weight of the load. The variation in the resonant frequency requires a calibration of the system during installation, or a reconfiguration, to compensate for a different weight of the load. It would be desirable to provide an elastomeric mount which has a resonant frequency that is relatively constant for a predetermined range of load weights.
U.S. Pat. No. 5,660,255 issued to Schubert et al. discloses a vibration isolator which has a number of piezoelectric actuators to isolate a load in a vertical direction and additional piezoelectric actuators to isolate the load in a horizontal plane. The Schubert vibration isolator provides active isolation in both the vertical and horizontal directions. The piezoelectric actuators are relatively expensive. Therefore providing additional horizontal actuators increases the cost of assembling the vibration isolator. It would be desirable to have effective vibration isolators that can provide vertical and horizontal isolation, and which cost less to produce than isolators of the prior art.
Even with vibration isolation the load may move relative to the floor in the horizontal plane. It may be desirable to move and adjust the load to an original reference position. It would therefore be desirable to provide a docking system which can align and secure the load in a reference position.
The drive signal which excites the piezoelectric actuator is typically a function of a gain value and a transfer function which are either stored in a memory device of a controller that controls the system, or built into analog electronics that control the system. The stored transfer function determines the transient response time and bandwidth of the isolator. Vibration isolators of the prior art do not allow the system operator to vary the transfer function and the resultant transient response time and bandwidth of the system. It would be desirable to provide a vibration isolator which allows an operator to vary the transfer function used to determine the drive signal of the actuator.